The central hypothesis of this proposal is that the immune system of the cancer-bearing host can be induced to respond to tissue-specific (differentiation) autoantigens. We propose that immune responses to tissue-specific antigens can mediate tumor rejection. A corollary to this proposition is that tumor cells are normally unable to present antigens effectively to the immune system. As a model, we propose to study the response to gp75, an abundant tissue-specific antigen whose expression is absolutely restricted to normal melanocytes and melanoma cells. The gp75 antigen, the gene product of the brown locus, is a transmembrane glycoprotein expressed in melanosomes. The gp75 antigen can be recognized by the immune system, and thus is a potential target for immunotherapy of melanoma. In our initial studies, we will examine methods to immunize against gp75 in a syngeneic mouse system (C57BL mice) using: 1) gp75+ melanoma cells; 2) gp75 melanoma cells expressing transfected or transduced gp75 or truncated, unstable constructs of gp75; 3) gp75+ melanoma cells secreting interleukin-2 or interferon-gamma (transduced by retroviral vectors); 4) purified gp75 protein and peptide fragments of gp75; and 5) gp75 expressed by or presented by antigen-presenting cells, including macrophages and dendritic cells. We will model immune responses to tumor-specific antigens by introducing mutations into the gp75 gene to construct potentially immunogenic T cell and B cell epitopes. We propose that immune responses generated against immunogenic tumor-specific epitopes (e.g. against mutations) can lead to immune responses both against the tumor-specific determinant and against native, non-mutant differentiation antigens expressed by parental tumor cells. The ability of immune responses against gp75 or mutated gp75 to reject melanoma tumors will be investigated. We will use a transplantable melanoma tumor and melanomas developing de novo in a transgenic mouse system. These studies will give insights into immune recognition of differentiation and tumor-specific antigens expressed by cancer cells and provide strategies for construction of cancer vaccines.